I. Field of the Invention
The invention generally relates to cellular telephone systems and in particular to a Code Division Multiple Access (CDMA) cellular system employing a power control system for minimizing power usage within a forward link.
II. Description of the Related Art
The forward link of a cellular telephone transmission system between a base station and a mobile station is illustrated symbolically in FIG. 1. More specifically, FIG. 1 illustrates a base station 10 and a mobile station 12 with mobile station 12 moving relative to base station 10. Base station 10 transmits signals on a forward link 13 to mobile station 12. Mobile station 12 transmits signals on a reverse link 15 to base station 10.
The amount of power needed to reliably transmit signals from base station 10 to mobile station 12 is affected by various factors including the distance between mobile station 12 and base station 10, shadowing, fading, and interference from other sources such as other cellular base stations (not separately shown). In FIG. 1, factors producing shadowing, fading, or interference are generally represented as noise sources 14. As a result of these and other factors, the minimum amount of power required to reliably transmit signals from base station 10 to mobile station 12 can vary considerably and in a generally unpredictable manner as a function of time.
FIG. 2A provides an illustration of an exemplary minimum required power curve 16 as a function of time in arbitrary power units needed for reliably transmitting signals, perhaps in the form of frames, from base station 10 (FIG. 1) to mobile station 12 (also FIG. 1). As can be seen, the minimum amount of power required varies considerably with time. FIG. 2A also illustrates a constant transmit power level 17. The minimum required power 16 exceeds the transmit power 17 between points 18 and 19 resulting in signal loss likely causing a frame erasure wherein signals comprising an entire frame of transmitted data are discarded or otherwise ignored by mobile station 12. Ideally, a perfect feedback system would be provided between mobile station 12 (FIG. 1) and base station 10 for allowing base station 10 to always transmit signals to mobile station 12 at precisely the minimum necessary power level 16 (FIG. 2A) to thereby ensure that each transmitted signal is reliably received while also ensuring that the amount of power transmitted is minimized.
In a CDMA communication system, capacity of the system is maximized when the transmit power is minimized because transmissions to one user are seen as noise to all other users. An exemplary embodiment of a CDMA communication system is described in U.S. Pat. No. 4,901,307 entitled "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters" and in U.S. Pat. No. 5,103,459 entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System" both of which are assigned to the assignee of the present invention and are incorporated by reference herein.
As a practical matter, however, it is difficult or impossible to provide a perfect feedback system which would allow base station 10 to always transmit signals at the minimum necessary power level. Hence, either some amount of signal loss must be tolerated or some amount of power excess, or both, must be tolerated. For some cellular systems a maximum average signal loss of 1%, as measured by a frame error rate (FER), is considered acceptable. Only short periods of time with an FER above 1% are tolerated. One method for setting the power level such that a predetermined FER is achieved is by having mobile station 12 transmit back a message each time that it detects a frame erasure. In response to the frame erasure message base station 10 may, for example, increase its transmission power by 1 db. Such a system is described in detail in U.S. Pat. No. 5,056,109 entitled "Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Telephone System", assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 2B illustrates one exemplary power transmission feedback control technique. Within FIG. 2B, a minimum power requirement curve as a function of time is identified by reference numeral 20. A curve illustrating the actual power transmitted by base station 10 (FIG. 1) is identified by reference numeral 22. With the system of FIG. 2B, the amount of transmit power is initially set by base station 10 to a high default value 23. The transmit power is then successively, incrementally reduced by base station 10, perhaps on a frame by frame basis, until a point 25 wherein the power drops below the minimum power required resulting in a frame erasure. Mobile station 12 transmits a frame error message signal (shown in FIG. 1 as reverse link 15) to base station 10 indicating that one or more frame erasures have occurred and thereby indicating that the transmit power needs to be increased. Thereafter, base station 10 significantly increases the level or gain of transmit power 22 to ensure that subsequent frames of the transmitted signal are not also erased.
In FIG. 2B, for clarity and simplicity in illustrating the feedback concept, individual frames are not shown. Also, a feedback delay between the time when the transmit power falls below the minimum required power and the time when the transmit power is increased is shown as being minimal (as indicated by only brief periods of time when the transmitted signal level 22 falls below the minimum required signal level 20). In practical systems, this feedback delay time may be more significant. Also, in some practical systems the feedback signal provided to the base station indicates that the FER exceeds some predetermined maximum for some predetermined period of time rather than indicating that one or more frame erasures has occurred. In other systems, the feedback signal identifies only two or more consecutive frame erasures. Herein, unless otherwise noted, it is assumed that the feedback signal identifies that at least one frame erasure has occurred.
After the power has been significantly increased, base station 10 (FIG. 1) incrementally decreases the transmit power during time period 27 (FIG. 2B) until yet another frame erasure occurs at point 29 triggering yet another significant increase in the transmit power level. As can be seen, the actual transmit power 22 follows a generally saw-toothed pattern defined by sharp increases in transmit power followed by gradual successive decreases in transmit power. By decreasing the transmit power in this manner, the system can thereby transmit less power then would otherwise be required if the system continuously transmitted signals at the initial high default power level 23.
In addition, the ratio between the transmit power decrease and transmit power increase can be selected in accordance with the desired FER. For example, if a 1% FER is desired, then the ratio between the magnitude of the power decrease to the magnitude of the power increase should be roughly equal to 1/100.
Although the technique of FIG. 2B is effective in reducing the overall power transmission requirements, it is not entirely satisfactory. In particular, the combination of sharp increases in transmit power followed by slow incremental decreases in transmit power level results in a relatively high average transmit power level compared to the minimum average required power. This is seen most easily in circumstances where the minimum required power remains essentially constant as illustrated in FIG. 2C. More specifically, FIG. 2C illustrates a constant minimum required power level 24 and a resulting sawtooth power transmission pattern 26 occurring upon an application of the foregoing feedback technique.
The amount of excess transmit power is particularly significant in circumstances where the minimum required power level remains relatively low but includes occasional peaks of higher power requirements, caused perhaps as a result of movement of mobile station relative to buildings or the like. In FIG. 2D, such a power requirement curve is shown and is identified by reference numeral 30. The power requirement curve includes brief periods of higher power requirements 34 and 36. The resulting actual transmit curve is identified by reference 32. As can be seen, the amount of transmit power increases significantly during the brief periods of higher power requirements 34 and 36. Immediately following those periods, transmit power 32 decreases relatively slowly and incrementally over relative long periods of time 35 and 37, respectively, until eventually falling below the minimum required power level 30 and then being increased again. During the periods of time (35 and 37) following the increased power requirements, the amount of transmit power 32 remains significantly above the minimum required power level 30 resulting in a significant excess of transmit power.
In view of these disadvantages with the aforementioned feedback power control techniques, it would be desirable to provide an improved system which reduces the average power transmission requirements and, in particular, which reduces the surplusage of transmit power following brief peaks in minimum power requirements. It is to these ends that aspects of the present invention are drawn.